Why such an extremely low level of moisture? Yes, a dryer can be designed and operated to meet the requirement. I recently worked on an 10 SCFM application where the operator wanted a dew point of -110☏. Most importantly, I must explain why a filter cannot remove water vapor. Some old school operators are still around who’ll say things like, “I need a filter to dry my compressed air stream.” This is where I need to deliver a quick lecture on dew point, humidity, and why dry air on the suction side of compressor comes out saturated. To be sure, in some applications a low dew point is justified, but over-drying compressed air is costly and wasteful. Many times compressed air does not need a dew point of minus anything, especially when the plant is operating routine equipment, tools, and cylinders in a climate-controlled environment. Sometimes, an ambitious customer will want to get the air “bone dry.” What does that mean? If that means having a -40☏ pressure dew point then a careful conversation needs to occur. This ISO standard is readily available online.įor many compressed air systems, simply preventing condensation is adequate. In standard 8573.1, various dew points classes are defined, and a relatively precise moisture level may be specified in a range from -94F-70C to +50F/+10C. The other group that offers a standard for dryness is the International Standards Organization (ISO). I won’t bore you here with the details, but will gladly do that upon request, as I am an acknowledged authority in the field of boring people. Many people consider the ISA “instrument air” standard as requiring a pressure dew point of -40 ☏, but I think this is due to a paragraph that shows -40☏ only as an example, and it gets mistaken for the standard itself. It defined dry air as having a pressure dew point 18☏ / 10☌ below the lowest temperature the system would encounter, but not higher than 35☏. One of the first published standards was from the Instrument Society of America (ISA) in standard S7.3. This moved many conversations from the realm of opinion to the realm of measurement. Over time, industry groups began to formally define and quantify the meaning of “dry” compressed air. For a few critical applications, you’d hear customers talk about dew point, but this was rare. If there was not visible liquid, all was well. Indeed many air system operators still insist that the absence of liquid signals a dry compressed air system.įor a long time there were no official standards of dryness and few people actually measured moisture content. As pneumatic equipment got more sophisticated and sensitive, the phrase “dry air” came to mean simply no visible liquid in the compressed air system. In Jurassic times, having “dry air” meant installing a separator or drop leg to remove enough liquid so that you’d be operating an air system rather than a high pressure water line. But none is completely helpful for understanding what it means to have dry compressed air. The dictionary lists a lot of definitions for dry. If you can’t see water in compressed air, does that mean it’s dry? This is not a trick question, but the answer does depend on the meaning of the word dry.
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